In April, Frontier Airlines was the latest carrier to announce an "unbundling" of the many extra fees that fliers now pay—for everything from checked bags to seat selection to on-board snacks. Now, one website is trying to help travelers re-bundle those charges to see what they'll actually be paying once all the nickels and dimes are added up.

Hopper's new Fee Calculator lets users enter a city pair—say, Boston to San Francisco—and then tick boxes for each extra they plan to purchase. Select, for example, one checked bag, a carry-on, a snack, Wi-Fi, and seat selection, and Hopper reveals that while American, Delta, and United have lower advertised fares, you'll actually get a better deal on JetBlue after you account for all the ancillary fees that would be tacked on by the bigger carriers. (Note, though, that JetBlue currently only offers Wi-Fi "on a very limited number" of planes.)

A similar search for flights from Los Angeles to Las Vegas reveals almost complete pricing parity across major airlines—except for Spirit, which at first glance appears to offer a much cheaper flight. But add in that carrier's charge for a carry-on bag larger than would fit under a seat—something many people reasonably travel with—and flying Spirit ends up costing almost exactly the same as you'd pay on competing airlines.

The one downside to Hopper's tool is that its totals are based on average aggregate airfare prices from the previous four weeks; the actual price of an individual ticket on a given flight may be different than what's shown. That means you can't do a spot-check of your exact itinerary. Even so, the calculator will help travelers baffled by seemingly low advertised prices make sense of an ever-expanding universe of extra fees.